DURABILITY OF PLYWOOD JOINTS MADE 

WITH ONE MELAMIME-, ONE UREA-, AND 

FIVE RESORCINOL-RESIN GLUES 

October 1947 

This Report is One of a Series 

Issued In Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1568 

UNlTED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

M a d i s o n 5 , Wisconsin 

In Cooperation with the Univeraity of Wisconsin 
DURABILITY OF PLYWOOD JOINTS MADE WITH ONE MELAMINE--, 

1 

ONE UREA- AND FIVE RESORCINOL-RESIN GLUES 

By W. Z. OLSON, Technologist 

and R. F. BLOMQUIST, Chemist 

Forest Products Laboratory Report No. 1530 (revised December 1946) summarizes durability data obtained at the Laboratory on several types of glues. 

That report includes data on only two resercinol-resin glues and no data on 

acid-catalyzed melamine-resin glues or noncrazing urea-resin glues. The Laboratory was requested by the Army-Navy-Civil Committee on Aircraft Design 

Criteria to augment the information on resorcinol glues and also to obtain 

data on the durability of catalyzed melamine-resin glue and of noncrazing 

urea-resin glue. 

Seven glues were selected for this study. Five of these were resorcinol-resin glues; one, an acid-catalyzed melamine-resin glue; and one, a 

urea-resinglue which was reportedly formulated to impart craze resistance 

to the glue joint. Six controlled exposure conditions were used, in addition 

to outdoor exposure. This report describes the preparation of test specimens 

and includes test data obtained over an exposure period of 18 months. The 

study is scheduled to continue for an additional 42 months. 

Preparation and Exposure of Birch Plywood Test-Specimens 

Six, three-ply, 25- by 25-inch panels were prepared with each of the 

seven glues, using 1/16-inch yellow birch veneers. The veneers contained about 

12 percent moisture at the time of gluing and were glued under the conditions 

given in table 1. After conditioning for at least 7 days at 80° F. and 65 percent relative humidity, 140 standard plywood shear test specimens were cut from 

each panel, producing 840 specimens for each glue. Sixty specimens with each 

glue representing all 6 panels were tested as controls, 30 dry and 30 wet after 

soaking for 48 hours in water at room temperatures. The remaining specimens 

were placed in the following exposures, to be tested at intervals of 2, 4, 6, 

9, 12, 18, 24, 30, 36, 42, 48, 54, and 60 months. 

1. Continuously in air at, 80° F. and 65 percent relative humidity. 

2. Continuously in air at 80° F. and 97 percent relative humidity. 

3. Continuously in air at 158° F. and 20 percent relative humidity. 

4. Continuously in water at room temperatures. 

1 This is one of a series of progress reports prepared by the Forest Products 

Laboratory relating to the use of wood in aircraft. Results here reported 

are preliminary and may be revised as additional data become available. 

Report No. 1568 -1-
12, 

5. A repeating cycle consisting of 2 weeks in air at 80° F. 

and 97 percent relative humidity, followed by 2 weeks 

in air at 80° F. and 30 percent relative humidity. 

6. A repeating cycle consisting of 2 days in water at room 

temperatures followed by 12 days in air at 80° F. and 

30 percent relative humidity. 

After each exposure interval, five specimens were tested immediately and 

after exposures 1, 3, and 6, five additional specimens were tested wet after 

48 hours soaking in water at room temperature. The specimens from exposure 

6 were tested wet after the soaking part of the cycle. 

Preparation and Exposure of Test Fence-Panels 

Five-ply, yellow birch and Douglas-fir plywood panels were prepared 

from veneer 1/8- by 25- by 25 inches in size. The veneers contained about 

12 percent moisture at the time of gluing and were glued under the same conditions as were the panels made for the plywood specimens. Two panels of 

each species were prepared with each glue. After conditioning for at least 

7 days at 80° F. and 65 percent relative humidity, the panels were trimmed 

to 18 by 18 inches in area and placed on outdoor exposure racks near Madison. 

The panels were unprotected by any coatings and were mounted vertically in 

an open space facing south. The panels were examined for delamination after 

10 months of exposure. 

The trimmings from the panels were cut into standard plywood shear 

test specimens with saw cuts made to test the inner glue lines. Five specimens from each panel were tested dry, five tested wet after 48 hours soaking 

in water at room temperature, and five tested wet after the soaking-drying 

cycle prescribed for high moisture resistant bondage in "Commercial Standard 

CS35-42 for Hardwood and Eastern Red Cedar Plywood." This test involved 

soaking the specimens for 16 hours in water at room temperature and drying 

for 8 hours at 145° F. (plus or minus 5° F.), repeating for a total of three 

cycles, soaking the specimens for an additional 16 hours at room temperature, 

and then testing while wet. Average results of these tests are given in 

table 2. 

Results 

Average results of shear tests made on the plywood specinens that were 

under controlled exposure conditions are given in table 3. The results were 

obtained at 0, 2, 4, 6, 9,/ and 18 months of exposure. 

Eighteen months of exposure was not long enough to permit drawing 

final conc1usions concerning ultimate durability, but the following trends 

were evident in the data for the 18-month period. 

Report No. 1568 -2-
The five resorcinol-resin glues generally showed similarity in 

behavior, There appeared to be some superiority in Glues C and E in maintaining very high wood failures, but there was no consistent difference in 

shear strength between the resorcinol-resin glues. 

Continuous exposure at 158° F, and 20 percent relative humidity 

proved to be the most damaging condition for the plywood glued with 

resorcinol resin. The shear strength might be expected to fall in this 

exposure because of possible heat damage to the wood. The average wood 

failure in the dry tests of the joints glued with three of the resorcinolresin glues, however, decreased appreciably, 

In the other exposures, some of the joints made with resorcinol-resin 

glue decreased in shear strength while maintaining relatively high percentages 

of wood failure. It appeared, however, that this decrease in shear strength 

resulted from the effect of the exposure conditions on the strength properties of the wood rather than from deterioration of the glue itself. The data 

on two resorcinol-resin glues given in Report No. 1530 agree closely with 

that obtained on the five resorcinol-resin glues of this study. 

Joints made with the acid-catalyzed melamine-resin glue generally

showed somewhat greater deterioration than those made with the resorcinolresin glues. Marked decreases in shear strength or wood failure, and 

usually in both, were observed when the specimens were tested after exposure 

to the more severe conditions. Slight decreases in shear strength and wood 

failure were also observed upon testing those specimens that were exposed 

continuously for 18 months in air at 80° F. and 65 percent relative humidity. 

Comparison with the data presented in Report No. 1530 tends to indicate this 

acid-catalyzed melamine-resin glue to be less durable than the average more 

neutral, hot-press melamine-resin glue. 

Joints made with the urea-rosin glue deteriorated to a much greater 

extent than did joints made with the other glues used in this study. 

Exposure to high temperature and to high humidity was particularly injurious 

to the joints in specimens made with urea-resin glue, A marked decrease in 

wood failure resulted even in the mild exposure of 80° F. and 65 percent 

relative humidity. Of the plywood panels exposed to the weather, only the 

birch panels glued with the urea-resin glue G developed visible delamination 

in the first 10 months (table 2). Comparisons with the data. presented in 

Report No. 1530 show Glue G to be generally about as durable as the average 

urea-resin glue which meets the requirements of Specification AN-G-8. 

Report No. 1568 -3-
Table 1.--Gluing conditions used in preparation of plywood panels 

The temperature of the room varied between 75° and 90° F. during 

the pressure period. 

Report No. 1568 

1 
Table 2.--Average results of initial tests of specimens from panels prepared 

for outdoor exposure and inspection of panels after 10 months' 

exposure to the weather 

1Tested dry. 

2Tested wet after soaking in water at room temperature for 48 hours. 

3Tested wet after soaking-drying cycle described for high moisture resistant 

bondage in Commercial Standard CS35-42 for Hardwood and Eastern Red Cedar 

Plywood. 

4The average for two panels. 

5 The value before the dash is the average shear strength in pounds per square 

inch; the value after the dash is the average wood failure in percent. Each 

value represents an average of 10 specimens, 5 from each of 2 panels. 

Report No, 1568